Biological treatment of wastewaters for removal of the chemical oxygen demand (COD) produces a biomass. Today, the wasted surplus biomass represents a solid waste disposal problem. An opportunity that has attracted much interest is the production of biodegradable polymers by biomass, such as that in activated sludge treating wastewater. In this way, at least part of the produced activated sludge becomes a valuable by-product that can be harvested from a biological wastewater treatment process. The harvested biomass can be made to accumulate significant levels of biopolymers and the now biopolymer-rich biomass is no longer a disposal problem but is, to the contrary, a raw material resource in the value chain towards bioplastics and/or fine chemicals. In this manner a sludge disposal problem may be turned into a renewable resource opportunity.
It is known that biomass produced in biological treatment of process waters and wastewaters can be made to accumulate biopolymers in the class of polyhydroxyalkanoates (PHAs), a group of polyesters produced by many naturally occurring species of bacteria as intermediate carbon and energy reservoirs. PHAs are biopolymers that can be recovered from biomass and converted into biodegradable plastics of commercial value that are useful for a broad range of practical applications (see for examples, US 2010/0200498, WO 2011/070544A2, WO 2011/073744A1, WO 2012/022998A1, WO 2012/023114A1).
From the body of academic and intellectual property publications (see for example Salehizadeh and Loosdrecht, Biotechnology Advances 22 (2004) 261-279), it is known that a means for enhancing the potential of biomass to accumulate PHA while treating a wastewater involves a so-called feast and famine strategy. Feast and famine means that the biological treatment is carried out such that the biomass is exposed to alternating environments of available and scarcely available substrate in the form of readily biodegradable COD (RBCOD). RBCOD may, for example, include volatile fatty acids such as acetic acid. One gram mass of acetic acid is equivalent to 1.067 grams of acetic acid as chemical oxygen demand or COD.
Under the conditions of suddenly available RBCOD during feast, RBCOD is taken up by the biomass just after being exposed to a famine environment. During feast, at least some RBCOD is converted into PHA. Under famine conditions with low RBCOD availability, the populations of bacteria in the biomass that stored at least some PHA during feast can use this internally stored PHA as a source of energy and carbon for growth and survival during famine. Thus the alternation of feast and famine environments tends to select for the survival of populations of bacteria in the biomass with ability to store PHA.
By applying feast and famine conditions, the PHA accumulation potential (PAP) of biomass can be enriched compared to typical minimal background levels found in biomass in conventional biological wastewater treatment systems. A typical minimal background PAP for activated sludge is an ability to store PHA to a level less than about 20% g-PHA/g-VSS. An enriched PAP may be considered to be an accumulation potential of about 30% or more, and preferably more than 50% g-PHA/g-VSS. A high PAP makes the accumulation process and subsequent recovery of PHA more efficient and thereby improves the overall process economy of producing PHA as a by-product of services in water quality management by biological treatment.
Production of PHA from services of water quality management can be a part of an overall biorefinery concept involving biological treatment unit processes comprising but not limited to:                1. Optional pretreatment such as acidogenic fermentation in order to convert organic matter into RBCOD fermentation products such as volatile fatty acids (VFAs).        2. Removal of organic contamination from the water and production of a biomass with potential for significant accumulation of PHAs or enriched PAP.        3. Expressing the PAP of the surplus biomass from 2, by controlled accumulation of PHAs in the harvested biomass by using RBCOD coming from either the same source as used for biomass production or by using other available sources of RBCOD.        4. Recovery and purification of the PHAs from the PHA-rich biomass produced in 3.        
Typical feast and famine selection of PAP has focused on maintaining stable conditions in time of a cyclic regime of feast and famine. This is to say that the biomass is exposed to repeated cycles of feast and famine, where the total feast-famine cycle time is approximately constant, and where the famine portion is generally meant to be greater than ¾of this cycle time. Although feast and famine selection has been repeatedly shown to enrich the PAP of a biomass produced in open mixed-cultures, results also suggest that the microbial community that comprises the biomass may adapt itself to the regime of feast-famine. By adaptation the microbial community may be less likely to directly reach its full potential of PAP during a PHA accumulation process.